The isometric muscular strength and endurance of two groups of ten volunteers were measured four times per day for 15 successive days. Eight subjects in one group were inoculated with sandfly fever virus on the seventh day of the 15-day experimental period and two were given sterile saline. All 10 subjects in the other group served as healthy controls. The measurements of muscle function were obtained as an incidental portion of previously reported studies concerning the effects of a mild virus illness on human work performance and responsiveness to symptomatic therapy. The virus-inoculated subjects experienced decrements in muscular strength and endurance which were significantly correlated with increases both in rectal temperature and in heart rate during the brief period of illness. Endurance was depressed to a greater extent than was strength, and the typical relative-load endurance invariance did not hold during illness.